The cerebellum (CB), consisting of 80% of the neurons in the human brain, not only has a major role in balance and motor coordination, but also modulates language, reasoning and social processes via neural circuits that connect throughout the forebrain. The ratio of the number of neurons in the CB to the cerebral cortex is remarkably constant across mammalian species, indicating that interconnected circuits have scaled together during evolution. Since much of CB growth occurs in the third trimester and continues for a year after birth, the CB is particularly vulnerable to clinical and environmental factors. Granule cell production, stimulated Sonic Hedgehog (Shh) secreted by Purkinje cells, accounts for a majority of cerebellar growth during this period. Pre-term babies are at a significantly higher risk of developing cerebellar hypoplasia and neurological dysfunction, likely in part because they receive glucocorticoids. In rodent models, glucocorticoids increase death of granule cell precursors (GCPs) in the external granule cell layer (EGL), through a mechanism that involves altered SHH-GLI signaling. However, our preliminary results and other experimental models have demonstrated that the developing rodent CB has a large capacity to regenerate a depleted EGL. In order to enhance recovery from a transient insult to the developing CB, it is critical to identify the signaling pathways that stimulate compensatory expansion of cells, and ensure that all cell types scale together in order to have normally functioning circuits. We will utilize sophisticated mouse genetics approaches to identify such pathways. Our studies are based on the unique discovery we made that when the anterior EGL of the CB is depleted at birth, cells marked with Nestin-FlpoER in the white matter expand and populate the EGL and then differentiate, producing a major recovery. Our preliminary studies and novel genetic approaches provide a powerful approach for studying the cellular behaviors of normal and mutant Nestin-expressing white matter stem cells in response to depletion of the EGL and discovering the signals that stimulate expansion of white matter stem cells and their population of the EGL. Our studies should provide insights that can be used to develop new approaches for augmenting recovery of the infant CB in the face of premature birth, glucocorticoid treatment or other injuries including hemorrhage. Our specific aims are: Aim 1. To determine the regenerative potential of cerebellar white matter Nestin+ stem cells. Aim 2. To identify signaling pathways which are altered in Nestin+ stem cells during regeneration of the EGL. Aim 3. To test whether SHH signaling or some of the identified pathways enhance recruitment of Nestin+ stem cells to a depleted EGL.